A slow adsorption of oxygen on MgO and CaO powders is enhanced after γ- or neutronirradiation; e.s.r. studies have indicated that F centers contribute to the adsorption on neutronirradiated material, with the F-centre decay and adsorption processes both obeying Elovich kinetics. Annealing of irradiated MgO has shown that the F centres affected by oxygen are also those which are thermally least stable and presumably correspond to those defects which are closet to the oxide surface. Adsorption of oxygen leads to the annihilation of the F centre defect rather than loss of the trapped electron and the potential barrier thus produced controls the rate of the subsequent reaction. Oxygen is adsorbed on unirradiated, γ-irradiated and neutron-irradiated SrO powders with diffusion-controlled kinetics. F centres play only a minor role in neutron-irradiated SrO but allow the estimation of diffusion coefficients of adsorbed oxygen species in the oxide. A fast initial adsorption of oxygen involves only the surface regions.